1. Field of Invention
This invention relates to communications spectrum allocation and reuse on a non-interference basis in bands which have pre-existing spectrum users (both transmit/receive type and receive-only type).
2. Description of Prior Art
Communication systems commonly use methods to optimize the use of the spectrum. There are several approaches involving radio networks where channels are selected to optimize system capacity.
Cellular phone and other types of systems use low power transmissions and a cellular architecture that enables spectrum to be reused many times in a metropolitan area. These systems assume that within the allocated frequency band, the system is the primary user and that there is a control or signaling channel between all nodes. The goal of these systems is to maximize the number of calls system wide given a fixed amount of bandwidth. This problem is complex because of the nearly innumerable choices of frequency/channel combinations possible, the time varying nature of the calls, and the unpredictable propagation loses between all of the nodes. While global optimization schemes would give the highest capacities, limited communications capacity between the nodes, finite channel measuring capabilities in some of the nodes, and short decisions times require that distributed non-optimal methods be used. Examples are disclosed in U.S. Pat. Nos. 4,672,657 (1987), 4,736,453 (1988), 4,783,780 (1988), 4,878,238 (1989), 4,881,271 (1989), 4,977,612 (1990), 5,093,927 (1992), 5,203,012 (1993), 5,179,722 (1993), 5,239,676 (1993), 5,276,908 (1994), 5,375,123 (1994), 5,497,505 (1996), 5,608,727 (1997), 5,822,686 (1998), 5,828,948 (1998), 5,850,605 (1998), 5,943,622 (1999), 6,044090 (2000), and 6,049,717 (2000).
The above patents describe methods where current channel measurements (noise level, carrier-to-interference ratio (C/I)), previous channel measurement statistics, and traffic loading are used in different ways to optimize capacity while minimizing latency in channel assignment, equipment requirements, and dropped calls. All of these methods assume that the system is the primary spectrum user. This would allow the primary system to select channels where it was jammed, but it would create significant interference to another system.
Several methods to enable a system to operate as the secondary spectrum user with minimal impact to the primary user have been disclosed. The first type assume that there are predetermined spatial “exclusions zones” where if the secondary user avoids transmission while located in these areas, then there will be no interference to the primary user. U.S. Pat. No. 5,422,930 (1995) uses a telephone circuit based keying method where the telephone's location is known and when the secondary user is connected to the specific phone line, authorization is given for operation using a set of frequencies. U.S. Pat. No. 5,511,233 (1996) is similar method where an undefined position location system is used. U.S. Pat. No. 5,794,1511 (1998) uses a GPS (global positioning system) to locate the secondary user.
This geolocation exclusion method has significant shortfalls. To determine the exclusion zones, propagation estimates or propagation methods would have to be made. There would be large uncertainties in the antenna type, antenna orientation, antenna height, and power level used by the secondary user. There would be uncertainties in the local propagation conditions between the secondary user and the primary user, and these propagation conditions might change because of ducting or other temporary atmospheric conditions. To mitigate these problems, the exclusion zones would have to have very large margins, which would greatly reduce system capacity, or some unintended interference would be created. These schemes do not address how the interference caused by one specific secondary user would be quickly and economically identified and eliminated.
A second type of secondary spectrum allocation method uses detailed propagation modeling of the primary and secondary communication systems and channel occupancy measurements made by the secondary system (U.S. Pat. No. 5,410,737 (1995) and U.S. Pat. No. 5,752,164 (1998)). The channel measurements are use to validate and improve the propagation modeling estimates. Using this information, the spectrum is allocated so that the primary user is not impacted.
Because of the large uncertainties in propagation estimates, the above method must use large margins to insure minimal interference. Using measurements of the propagation losses between the primary and secondary user can be directly used to reduce these margins only if the primary system transmits and receives using the same antenna, at the same frequency and at a known power level. In this case the secondary radio directly estimates it's impact on the primary system and can select its frequency and power level to avoid interference. However, most communication systems use different transmit and receive frequencies and often use different transmit and receive antennas. Hence, the measurements of the primary signal received by the secondary don't provide direct information on the impact the secondary transmitter has on the primary receiver. This method also doesn't describe how unintentional interference would be identified and mitigated.
A third approach insurers that the measurements of the primary signals made by the secondary user can be used to determine the available spectrum is to add a narrow bandwidth “marker” signal to every primary receiver antenna system (U.S. Pat. No. 5,412,658 (1995)). This approach has significant cost impact to the primary user and because the CW marker transmitter is collocated to the primary receiver, it will cause significant interference to the primary user.
A fourth method has the primary and secondary users sharing a spectrum band between the primary and secondary users to reserve bandwidth (U.S. Pat. No. 5,428,819 (1995)). An “etiquette” is observed between the users and each user makes measurements of the open channels to determine priority usage. This method has the disadvantage that the primary system must be modified to communicate with the secondary system, which is cost prohibitive if the primary user is already established. Also, the method will fail in many cases because of the well known “hidden node problem”. This occurs when the secondary nodes are unable to receive transmissions from a primary node because of the particular propagation conditions. Thus, the secondary user incorrectly believes the channel is available and his transmissions cause interference.
A fifth method assumes that the primary and secondary systems are controlled by a central controller (U.S. Pat. Nos. 5,040,238 (1991), 5,093,927 (1992), 5,142,691 (1992), and 5,247,701 (1993)). When interference occurs, the secondary system's power level and/or frequency list is adjusted. Some of the methods use channel measurements at the secondary system to detect changes in the frequency usage that would require a re-prioritization of channels. This method has obvious problems because the primary system would have to be highly modified to interact with the secondary system and to be able to make the required spectrum measurements. The spectrum is now fully allocated and there are primary users in every band. What is needed is a method that enables secondary operation without any modification to the existing primary user.
A sixth method uses field monitors the measure the secondary signal strength at specific locations. One sub-method is intended to enable secondary usage inside buildings (U.S. Pat. Nos. 5,548,809 (1996) and 5,655,217 (1997)). Field monitors are located surrounding the secondary system nodes which determine what channels are not used by nearby primary systems or if the channels are in use, if the coupling between the primary to them where the coupling to detected. The second sub-method is intended to enable adjacent cellular based mobile communication systems (U.S. Pat. Nos. 5,862,487 (1999)).